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Effect of Surface
Contamination on Composite
Bond Integrity and Durability
Dr. Benjamin Boesl
Florida International University
2
Effect of Surface Contamination on Composite
Bond Integrity and Durability Motivation and Key Issues
– There is significant interest in assessing the durability of composite bonded
joints and how durability is affected by contamination.
– Need to study mechanisms of failure to understand property influences and
possibly predict bond failure
– Past research has focused on determining/understanding acceptable performance
criteria using the initial bond strength of composite bonded systems.
Objective
– Formulate a methodology to create undesirable bonding conditions in a
scalable and repeatable manner.
– Evaluate methods to mitigate the undesirable conditions via surface preparation
methods.
– Investigate the effect of harsh environmental conditions on adhesive bonds
– Understand the mechanisms of fracture and failure of these bonds to further
improve predictive abilities.
– Support the CMH-17 Handbook
3
Principal Investigators
- Dwayne McDaniel, Ben Boesl
Students
- Brian Hernandez, Gabriela Gutierrez-Duran, Fernando Rojas, Mauricio Pajon
FAA Technical Monitor
- Ahmet Oztekin
Industry Participation
- Exponent, 3M, Embraer
Effect of Surface Contamination on
Composite Bond Integrity and Durability
4
Manufacturing of Bonded
Systems
KEY QUESTION
What happens to bonded
joint’s strength when
contamination occurs, if
known can it be mitigated?
CAUSES
Contamination can occur
in a manufacturing setting
from oil on hands, mold
release, leakage/spillage,
etc.
Fabrication of
Laminates
(Cure Cycle @350F)
Bonding of
Laminates
Preparing/Cutting
Samples
Laminate Cure
Adhesive CureAdhesive Bond
Strength Testing
(Cure Cycle @350F)
5
Materials
• Material type and curing procedure for specimens:
Unidirectional carbon-epoxy system, film adhesive, secondary curing
bonding and contaminants.
• Materials utilized:
• Toray P 2362W-19U-304 T800 Unidirectional Prepreg System (350F
cure)
• 3M AF 555 Structural adhesive film (7.5x2 mills, 350F cure)
• Precision Fabric polyester peel ply 60001
• Frekote 700-NC from Henkel Corporation
6
Contamination Approach
GOAL - Develop a process to create a scalable and repeatable weak bond
via bondline contamination.
Contaminant – Frekote release agent
• Developed a station that can uniformly spray contaminant – vary nozzle size and
spray rates
• Potential for creating a scalable weak bond by adjusting volume of Frekote
• Total amount of contaminate applied is measured using an analysis of pre- and
post- weight measurement.
7
Calibration of Contamination Levels
• Calibration of the contamination levels is important in order to be able to
trace back the amount of contaminant used and relate that amount to the
strength of the weak bond created
• This enables us to determine the different bond strengths that can be created
from different amounts of contaminant
• Adjusting spray speeds and mass measurements of the contaminant on a 1”
x 1” section of a panel, allows for the determination of the strength of the
weak bond
• Procedures
• Modify the spray speed according to the amount of mass desired
• Fast speeds: less mass
• Slow speeds: more mass
• Weigh a 1” x 1” section of a panel before spraying contaminant
• Spray contaminant and weigh it again
• Continue process until desired mass is reached
8
Bond Quality Evaluation
• Dual Cantilever Beam Testing
– Measures interlaminar fracture toughness
• Fracture toughness provides a measure of
composite strength
– The critical energy a material may absorb
before failure and resistance to
delamination
– 𝐺1𝐶 =3𝑃𝛿
2𝑏(𝑎+ ∆ )
• Use of MTS machine to measure displacement
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0 5 10 15
Lo
ad
(kN
)
Displacement (mm)
Baseline
Contaminated
9
Contamination Results
0.10 0.15 0.20 0.25
Contamination Mass (mg)
0.0
0.2
0.4
0.6
0.8
1.0
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0
G1
c(kJ/m
2)
Spray Speed (in/s)
70-80% Range 70-80% Range
45-55% Range 45-55% Range
0-20% Range 0-20% Range
10
Mitigation Procedures
• GOAL – Evaluate
mitigation processes that
are designed to remove
contamination of the
bondline
• Two methods of mitigation
– Solvent Wipe -
Attempt to remove
contaminate off of
surface with soaked
cloth
– Sanding of Material -
Actively remove
material using abrasive
Results of Mitigation Approaches
11
Baseline Cont Wipe WSW
19% 1.270 0.238 0.221 0.609
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8G
1C
(kJ/m
2)
0-20% of Baseline
Baseline Cont Wipe WSW
42% 1.320 0.554 0.575 0.869
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8G
1C
(kJ/m
2)
45-55% of Baseline
Baseline Cont Wipe WSW
78% 1.030 0.800 0.673 1.050
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8G
1C
(kJ/m
2)
70-80% of Baseline
12
Mitigation Results
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Baseline Cont Wipe WSW
G1
C(k
J/m
2)
19% 42% 78%
13
Failure Modes – 0-20% of Baseline
Baseline
19% Wipe
19% Wipe/Sand/Wipe
19% Only
Mixed-mode
failure
Variable
combination of
interlaminer
and cohesion
Adhesion
failure
Separates
from the
surface of
adherent
14
Failure Modes – 45-55% of Baseline
Baseline
42% Wipe
42% Wipe/Sand/Wipe
42% Only
Mixed-mode
failure
Variable
combination
of interlaminer
and cohesion
Adhesion
failure
Separates
from the
surface of
adherent
15
Failure Modes – 70-80% of Baseline
Baseline
78% Wipe
78% Wipe/Sand/Wipe
78% Only
Mixed-mode
failure
Variable
combination of
interlaminer
and cohesion
Adhesion
failure
Separates
from the
surface of
adherent
Environmental Aging
• Coupons were exposed to 70°C and
95% rel. humidity
• 8 coupons were manufactured for
each set: baseline, contaminated,
and wipe/sand/wipe
• 4 coupons from each set were
exposed in the environmental
chamber and the remaining 4
coupons served as the unexposed
set
• After 4 weeks in the environmental
chamber, the exposed samples were
removed from the chamber and
DCB tests were performed.
16
Results - Post Environmental Exposure
17
Unexposed Exposed
Baseline 0.892 0.734
Contaminated 0.065 0.091
WSW 0.288 0.232
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
G1
C(k
J/m
²)
0-20% of Baseline
Unexposed Exposed
Baseline 0.57 0.65
Contaminated 0.24 0.26
WSW 0.29 0.34
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
G1
C (
kJ/
m²)
45-55% of Baseline
18
In-situ Testing
Combined Load Frame and Electron
Microscopy
18
Test Development
µDCB (Dual Cantilever Beam)
Assess the mechanisms of
mode I fracture. Fixture was
designed based on literature of
metal-adhesive bond testing.
µENF (End Notch Flexure)
Assesses the mechanisms of
mode II fracture. Fixture was
designed based of traditional
ENF testing of composite
bonds
19
In-situ Testing
Combined Load Frame and Electron Microscopy
19
Baseline
Contaminated
Specimen Details Baseline
L/W: 40mm x 10mm
thickness: 5.2 mm
Pre-crack: 8 mm
10 layer unidirectional
composite panels
Observations
• Initially bond is very stiff
• Controlled crack propagation begins at ~50N Load
• Unstable crack growth begins at the pre-crack then
travels to composite-adhesive interface
20
In-situ Testing
Combined Load Frame and Electron Microscopy
Baseline
Contaminated
Specimen Details Contaminated
L/W: 40mm x 10mm
thickness: 5.2 mm
Pre-crack: 8 mm
4% contamination
procedure was used at
the interface
Observations
• Initial delamination between adhesive and composite
panel
• High compliance during loading, reduction in peak load
• Unstable crack growth begins at the interface and pre-
crack remains un-damaged
21
22
23
In-situ Testing
Combined Load Frame and Electron Microscopy
Complications with in situ testing
Small sample sizes and edge effects
Sample testing environment
At the moment, testing can be used
to study mechanisms but
not to quantify fracture properties
From Linear Elastic Fracture Mechanics
theory we know the stress field very near
the crack tip and from that we can solve
for the displacement at any point if
KI is known.
Therefore if we know the
displacements we
can solve for the KI value.
From LEFM
24
In-situ Testing
Combined Load Frame and Electron Microscopy
In situ Microscopy Digital Image Correlation Digitized Displacements
25
• A contamination procedure was developed using a customized contamination rig with Frekote to create a scalable weak bond. The weak bonds can be used to evaluate surface prep techniques and potentially NDI methods using three levels of contamination.
• Mitigation approaches included solvent wiping and solvent wiping/sanding/solvent wiping. Results from these tests indicated that wiping alone did not improve the bond strength, however, there was significant improvement with the wiping/sanding/solvent wiping.
• A platform for testing mini-DCBs has been established with the intent of quantifying fracture toughness via DIC strain mapping within an SEM. In addition, crack-tip propagation phenomena can be investigated to gain better understanding of the failure modes.
Summary
26
• Contaminated and treated DCB coupons will be fatigued in a hydraulic fatigue rig that can cyclically load specimens in shear via three point bending. After the specimens have been aged, effects of fatigue on the contaminated specimens will be evaluated.
• SEM image stabilization will be sought to improve accuracy of DIC techniques for fracture toughness quantification. Pre-crack propagation will be studied for detectable patterns in baseline and contaminated specimen.
Path Forward
27
Test SAMPLES
• 10 layers – Unidirectional Prepreg carbon fiber
(Toray)
• 3M AF555M film adhesive:
• 2 Layers
• 2 Layers contaminated
• 4 Layers
• 7mm by 70mm mini DCBs with 20mm
pre-crack
• Frekote Mold Release Agent (contaminant)
Baseline 2 layers
Baseline 2 layers
Baseline 4 layers
Baseline 4 layers
Baseline 2 layers (contaminated)
Baseline 2 layers (contaminated)
35
CMH-17 Support
Background and Motivation
• A Strategic Composite Plan has been developed by the FAA and has
identified three focus areas regarding safety, certification and
education. Within these areas, there are a number of initiatives related
to structural issues and adhesive bonding.
• As part of the FAA’s bonding initiatives, the CMH-17 handbook is supporting the development of content related to bonding design and process guidelines.
Mission Statement
The Composite Materials Handbook organization creates, publishes
and maintains proven, reliable engineering information and standards,
subjected to thorough technical review, to support the development
and use of composite materials and structures.
36
CMH-17 Bonding Process Task Group
Need for bonding process content in CMH
The Promise of Bonded Composites
lighter weight, monolithic structures
designed with fewer parts and
assembled with reduced
manufacturing costs (in terms of
time and labor)
The Reality of Bonded Composites
bonded parts that are bolted for
confidence, adhesives asked to act as
environment seals, challenges of
process control to capture and
quantify variability
Advantages Disadvantages
Bonded Joints
Small stress concentration in
adherends; stiff connection; Excellent
fatigue properties; No fretting
problems; Sealed against corrosion;
Smooth surface contour; Relatively
lightweight; Damage tolerant
Limits to thickness that can be joined
with simple joint configuration;
Inspection other than for gross flaws
difficult; Prone to environmental
degradation; Sensitive to peel and
through-thickness stresses; Residual
stress problems when joining to metals;
Cannot be disassembled; May require
costly tooling and facilities; Requires
high degree of quality control; May be
of environmental concern
Bolted Joints
Positive connection, low initial risk;
Can be disassembled; No thickness
limitations; Simple joint configuration;
Simple manufacturing process; Simple
inspection procedure; Not
environmentally sensitive; Provides
through-thickness reinforcement; Not
sensitive to peel stresses; No major
residual stress problem
Considerable stress concentration
Prone to fatigue cracking in metallic
component; Hole formation can
damage composite; Composites's
relatively poor bearing properties;
Prone to fretting in metal; Prone to
corrosion in metal; May require
extensive shimming
37
CMH-17 Bonding Process Task Group
Executive Summary
An outline for composite bonding processes was created and circulated for approval.
The CMH-17 Bonding Process Task Group used the outline as a framework to create
an online forum to capture organize and edit relevant content. The content in the
online forum will be converted into draft for circulation, editing and approval.
Bonding Process Task Group Leadership
Dwayne McDaniel FIU
Tanila Faria Embraer
Tim Barry BTG Labs
Dan Ruffner Emeritus
Howard Creel 3M
Bonding Process Task Group Champions
Curt Davies FAA
Rachael Andrulonis CMH-17
Bonding Process Task Group Steering
Nathan Weigand FAA
Bill Nickerson Navy
Michelle Johnson LMCO
Special Thanks to Founding Members
Holly Thomas, Margaret Roylance, Dan
Ruffner, Scott Leemans, Carl Rousseau
Bonding Process Task Group Sponsor
Margaret Roylance – M&P
38
CMH-17 Bonding Process Task Group
5.9 ASSEMBLY PROCESSES
Assembly processes are not conventionally covered within composite material
characterization, but can have a profound influence on the properties obtained in
service. As seen with test coupons, edge and hole quality can dramatically affect the
results obtained. While these effects are not usually covered as material properties, it
should be noted that there is an engineering trade off between part performance and
the time and effort expended toward edge and hole quality. These effects need to be
considered along with the base material properties.
5.9 Assembly Processes
5.9.1 Fastened Joints
5.9.2 Bonded Joints
39
CMH-17 Support
40
CMH-17 SupportUsing online forums to organize CMH-17 content